Warming could add to risk in drought

Apr 14, 2009      The Arizona Daily Star

Tom Beal

Apr. 14, 2009 (McClatchy-Tribune Regional News delivered by Newstex) -- Drought could kill trees at five times the historical rates if the global temperature rise predicted by scientists in the next century becomes reality.

That's one of the startling conclusions reached in a tree study conducted over the past two years inside the University of Arizona's Biosphere 2 complex near Oracle.

Pinon pine trees were placed in two separate areas of Biosphere 2, with a temperature difference of 4 degrees Celsius (7.2 degrees Fahrenheit).

Water was withheld from half the trees in each area, and the trees at higher temperature all died within 20 weeks. Those in slightly cooler air survived for up to 29 weeks.

"The bottom line is, 'Wow, this looks like it's going to be a very big effect,' " said David Breshears, a UA professor of natural resources and one of the co-authors of the study, published Monday in the Proceedings of the National Academy of Science.

Scientists had suspected that temperature alone could accelerate the die-off of trees during droughts, but this experiment is the first to quantify the rate in a controlled experiment with mature trees, said Henry Adams, the UA graduate student who conducted the experiment and wrote up the study with his two advisers -- Breshears and Travis Huxman, director of Biosphere 2.

The temperature difference chosen matches the global-warming prediction of a 2007 Nobel Prize-winning panel of scientists. That U.N.-sponsored group concluded that temperatures will rise about 5 to 9 degrees Fahrenheit by 2100 under a "business-as-usual" scenario, said Adams.

The UA study -- "Temperature sensitivity of drought-induced tree mortality portends increased regional die-off under global-change-type drought" -- also explored the mechanism by which the trees died, a little-understood phenomenon, said Breshears.

Adams said the experiment would have been difficult to conduct anywhere but Biosphere 2, a 7.2 million-cubic-foot sealed dome divided into regions that mimic different ecological zones.

The UA has managed the facility since June 2007.

"It's pretty amazing as a graduate student to get to work there. It was definitely a motivating factor in coming to the UA," said Adams, who is pursuing a doctorate in ecology and evolutionary biology.

Breshears said the experiment builds on earlier observational work done by him and others.

In one study, Breshears compared climate data and tree die-off during a drought in the Four Corners states in the 1950s with similar observations of more extensive die-offs in a drought that ended in 2003.

The more recent drought was no more severe, but temperatures were uniformly higher, Breshears said. Complicating observational studies was the presence of pine beetles during that drought, during which about 10 percent of the region's pinon pines died.

Another wide-ranging study of "background die-off" of trees in old-growth forests throughout the West suggested a similar link between temperature and accelerating tree death, Breshears said.

Observational studies go only so far, he said. "Biosphere 2, as a facility, creates an opportunity for us to go after some questions that are hard to answer in other settings," Breshears said.

For the experiment, researchers trucked in 50 pinon pines up to 6 feet tall from a ranch in New Mexico and randomly selected 20 thriving trees to place inside the glass-and-steel enclosure.

Ten trees, their roots contained in large pots, were placed in the savanna region, where temperatures approximated their native surroundings. Another 10 were placed in the desert, where temperatures were 7 degrees higher.

Five were watered and five were not in each setting.

The unwatered trees all died, but the ones at higher temperature died 28 percent faster. The watered trees remain alive.

Adams then compiled climate data on the historical occurrence of severe drought in the Four Corners region. He calculated that the trees' die-off would be five times the historical rate with the temperature rise.

"A fairly small difference in temperature makes a big difference in die-off," said Adams. A once-in-a-hundred-year event becomes once in 20 years."

Careful observation of the trees' response to drought also created a clearer picture of how trees die, said Adams.

To avoid losing moisture during drought, the trees close their stomata -- their pores, said Bre-shears. "They try to wait it out. They say, 'OK, let's not lose any more water,' " said Breshears.

When they close themselves off to that process, they use up their carbon stores and eventually starve to death. The warmer ones starve faster, said Adams.

Adams, Breshears and Huxman are already working on a follow-up study that will test what they learned in Biosphere 2, using pines kept at different altitudes outside Flagstaff.

It will be harder to control all the variables in that experiment, but it will more closely approximate natural conditions, Adams said.

Contact reporter Tom Beal at 573-4158 or tbeal@azstarnet.com.

Newstex ID: KRTB-0014-34135590

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